Conventional structures of energy storage devices including batteries such as lithium-ion batteries include wound electrode assemblies formed by alternately layering a positive electrode, a negative electrode, and two separators, then winding these sheets layered in this manner. In energy storage devices having such wound electrode assemblies, the two separators are fixed to a core at winding starting points at the center of the wound electrode assembly before the sheets are wound. The two separators are fixed, for example, by thermal bonding or with tape. Here, thermal bonding is preferred as a fixing method in terms of durability within the internal environment of the battery and impact battery performance. For example, Patent Literature 1 (PTL 1) discloses a wound battery in which an end of a separator is thermally bonded to an axis (core).